1. Field of the Invention
This invention relates to a semiconductor memory device and a manufacturing method thereof.
2. Related Art
Recently, FBCs (Floating Body Cells) have been expected to serve as memories alternative to 1T(Transistor)-1C(Capacitor) DRAMs. The FBC is provided with an FET (Field Effect Transistor) that has a floating body (hereinafter also, body) on an SOI (Silicon On Insulator) substrate, and stores therein data “1” or “0” depending on the number of majority carriers accumulated in the body. For example, when the number of majority carriers in the body is large, the data “1” is stored. When the number of majority carriers in the body is small, the data “0” is stored.
Bipolar writing is used as a method for writing data “1”. According to the bipolar writing, a forward bias is applied to a PN junction between an emitter layer and a drain layer and majority carriers are charged in a channel body. The emitter layer has the opposite conductivity type to that of the drain layer and is adjacent to the drain layer. The bipolar writing enables the data “1” to be written in memory cells at higher speed as compared to writing based on an impact ionization current.
However, bipolar-write FBCs have a unit cell size of about 16 F2 because emitter layers needs to be added. F indicates a size unit when the minimum pitch size of bit lines is 2 F and can be used as an indicator of the unit cell size regardless of the degree of downscaling. The unit cell size of ordinary FBCs without emitter layers is 4 F2 to 6 F2. Thus, the cell size of the bipolar-write FBC becomes an issue.